Communication devices having a direct conversion architecture may include mixers to down convert received radio frequency (RF) signals into baseband (BB) signals, and may include mixers to up convert BB signals into RF signals to be transmitted. With a different conversion architecture, the down conversion is from RF signals to intermediate frequency (IF) signals and then to BB signals, and the up conversion is from BB signals to IF signals into RF signals.
The mixers may use reference signals having a reference frequency similar to the carrier frequency of the received/transmitted RF signal. Generation of the reference signal using a local oscillator (LO) at a local oscillator frequency similar to the carrier frequency may lead to unwanted spurs and direct current (DC) offsets in direct conversion receivers. Therefore, some communication devices include a local oscillator to generate local oscillator signals at approximately one third of the desired reference frequency, and then use a frequency tripler to generate signals at the desired reference frequency from the local oscillator signals. However, a frequency tripler may consume a lot of power and may generate unwanted harmonics. Other communication devices include a square wave local oscillator to generate a local oscillator signal at approximately one third of the desired reference frequency, but the local oscillator signal has low third harmonic content and high content of other components.
Gilbert-cell active mixers are commonly used for fundamental conversion. For example, in down-conversion, the output of the mixer has a dominant spectral component at a frequency given by the difference of the local oscillator frequency and the carrier RF frequency. However, when used for third order sub-harmonic conversion, their loss is very high. In other words, the spectral component at three times the local oscillator frequency less the carrier RF frequency is very low.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.